Drilling tool

ABSTRACT

A drilling tool for drilling bores in solid metal, with a shank having at its front end several geometrically similar indexable inserts arranged at various radial distances and staggered with respect to each other by the same peripheral angle, whose working areas overlap, each of them having two equally long cutting edges, two of the respectively contiguous cutting edges inclined with respect to each other at an obtuse angle being simultaneously engaged, whereby the radial inner indexable insert extends slightly with one of its engaged cutting edges beyond the drill axis, at least one of the bisectors of the two pair set cutting edges of indexable inserts being inclined with respect to the longitudinal shank axis by an angle up to 25° m and at least one of the bisectors running at an angle differing from zero with respect to another bisector, so that the sum total differing from zero of the radial force component (F R ) resulting from all engaged effective cutting edges and the corresponding sum of the horizontal force components (F H ) form a general resultant (F RES ) acting upon the cutting edges and which is oriented in the direction of the radially utmost indexable insert at an angle (β) differing therefrom by maximum 45°.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a national phase of PCT/DE 91/00425 filed May 22,1991 and based upon a German application P 4018043.3 filed Jun. 6, 1990under the International Convention.

FIELD OF THE INVENTION

The invention relates to a drilling tool for drilling bores in solidmetal, with a shank having at its front end several indexable insertsarranged at various radial distances and staggered with respect to eachother by the same peripheral angle, their working areas overlapping,whereby the radially inner indexable insert slightly extends beyond thedrill axis with its engaged cutting edge.

BACKGROUND OF THE INVENTION

It is known to the state of the art that depending on the arrangement ofthe indexable inserts the asymmetrical cutting force conditions lead toan unfavorable radial action of the force on the drill shank, which cancause the drilling to deviate. For this reason, many times the operationis performed with one or more support rods fastened to the drill shank,which support the drilling tool against the bore walling. The drawbackis that the support rods are highly subjected to wear and besides cannot prevent the machine support of the drill from being exposed to aheavy load. Also surface quality of the bore is very defective.

Therefore the DE 27 30 418 C2 proposes a drilling tool wherein thebisector of the engaged cutting edge of each indexable insert isarranged parallelly to the drill axis. As cutting inserts, preferably,indexable inserts, are used, wherein at each second cutting edge theneighboring cutting edges define an angle of 156°. However, a forcebalancing of the drill shank in radial direction, which is supposed tobe accomplished by this drilling tool, is only then possible when eachtime both engaged cutting edges are basically engaged over their fulllength. In other words, the working areas are not allowed to overlap.Such structure has the disadvantage that when the diameter of the boreis changed, a new set of cutting inserts has to be used.

In order to eliminate this disadvantage, the EP 0 054 913 B1 proposesthat the mounting for the individual indexable inserts be arranged sothat the radial cutting force components of the engaged cutting edge arebalanced for each individual indexable insert, in that the bisectors ofthese cutting edges are inclined with respect to the parallel to thedrill axis by an angle which depends on the effective length of thecutting edge engaged along a partial segment.

In order to balance the radial forces, in DE 27 51 255 C2 it has alsobeen proposed to bend the radial plane of the inner cutting inserttowards the rear, against the rotating direction of the drill, by acertain angle with respect to the peripherally arranged cutting insert.However, as a result the forces are merely parallelly oriented, but notbalanced, since the angle of displacement can not have any influence onthe size of the radial or cutting forces. This way the forces aremaintained as a function of the cutting conditions and are independentfrom the geometry of the tool.

Finally in the DE 38 02 290 C1 dealing particularly with drillingthrough stacks of plates but also through a single piece solid metalmaterial, it has been proposed to arrange a first indexable insertradially outside on a diameter beyond the half defined by the drill axisand on the opposite side to arrange two smaller indexable inserts,whereby the effective outer cutting edge of the indexable insertarranged radially outside runs approximately on the same working coneand the same maximal radius as the radially outer cutting edge of thelargest indexable insert. This arrangement is also supposed to achievemerely a balance of the radial cutting forces.

OBJECT OF THE INVENTION

It is the object of the present invention to further develope theaforementioned drilling tool, so that all influences beyond the radialforce components be taken into account during drilling, with the purposeto cover the largest possible area of staggered bore diameters with aminimum of indexable insert sets, whereby scores are avoided in the borehole.

SUMMARY OF THE INVENTION

This problem is solved by a drilling tool according to the presentinvention. Compared to the aforementioned drilling tools according tothe state of the art, the difference resides in the conscious selectionof an indexable insert arrangement wherein the sum of the radial forcecomponents resulting from the total of effective cutting edges inengagement does not equal zero. According to the invention, theremaining radial force components form together with the sum of allhorizontal force components an overall resultant acting on the cuttingedge, oriented in the direction of the radially utmost indexable insertor at an angle differing by 45° from the latter, whereby bore holes canbe cut whose diameter is only slightly larger than the drill diameter.This offers the advantage that the drilling tools can not jam in thebore hole and that no retraction scores occur when the drill shank iswithdrawn. Thereby the inclination angle of at least the utmostindexable insert with respect to the longitudinal axis of the shankdepends firstly on the apex angle, respectively apex angles formed bythe effective cutting edge, respectively cutting edges of each indexableinsert, further on the degree of overlapping of the working areas of theused indexable inserts as well as the drilling diameter, and finallyalso on the mutual relationship between the inclination angle of thebisectors of the used indexable inserts. Experience has proven that withthe same apex angle of the used indexable inserts and with a fixed,preselected angle of inclination of the inner indexable insert, whichcan also be equal to zero, the inclination angle of the radially outerindexable insert has to be selected bigger when the drilling diameterdecreases. The same applies also when indexable inserts with larger apexangles are selected. With the same apex angle and the same drillingdiameter, the inclination angle of the inner indexable insert decreaseswhen a larger inclination angle is selected for the radially outerindexable insert.

While according to the aforementioned first solution each time twogeometrically similar indexable inserts each with two cutting edges ofequal length have been used, whereby the respective two contiguouscutting edges form together an obtuse apex angle, according to a secondembodiment, the use of at least one indexable insert with a trochoidalcutting edge is provided. For the inserting position of this trochoidalindexable insert the same applies with respect to the median axis ofsymmetry of this indexable insert as for the aforementioned bisector ofthe apex angle of an indexable insert with two equally long cuttingedges.

The apex angles of the equally long cutting edges of the used indexableinserts should be selected between 115° and 160°, preferably at 160°,156°, 145°, 125° or 115°. Depending on the apex angle, the inclinationangles of the bisector with respect to the longitudinal shank axis areselected so that with an apex angle of 160° the inclination angle of theinner, respectively outer indexable insert will be of maximum 8°. Withsmaller apex angles, e.g. 115°, the inclination angle, respectivelyinclination angles can be selected up to a maximum of 25°.

According to a further embodiment of the invention, the bisector or theaxis of symmetry of the cutting edge(s) of the inner indexable insert isinclined by a smaller angle with respect to the longitudinal shank axisthan the respective angle of the outer indexable insert. Hereby withclearly overlapping areas of the indexable inserts the particular caseis addressed when the inclination angle of the inner indexable insertequals zero, and the inclination angle of the outer indexable insertlies between 3° and 5°. When a total of two indexable inserts arearranged on the frontal end of the shank, it is further preferred thatthe inner indexable insert have two equally long cutting edges and theradially outer indexable insert use a trochoidal cutting edge. Theinclination angle of the axis of symmetry of the trochoidal cutting edgecan reach a maximum of 25°, and is preferably each time larger than theinclination angle of the bisector of the inner indexable insert.

Finally the drilling tool can be designed so that the indexable insertshave different sizes, respectively cutting edges of different length,seen from one indexable insert to the next indexable insert.

Preferably the length of the effective (engaged) cutting edge of theradial utmost indexable insert is by at a range of 8% to 15% longer thanthe length of the effective cutting edge of the radially innermostindexable insert. This is particularly the case where on one half of thedrill more than one indexable insert is mounted and whereby the lengthof the effective cutting edge of the indexable inserts increases frominnermost to the utmost indexable insert.

According to a further embodiment of the invention, the overlapping ofthe working areas of the cutting edges can be established also due tothe fact that the radially inner indexable insert protrudes by a maximumof 0.15 mm with respect to the other indexable inserts. Thereby duringthe start of the drilling operation a good centering of the toolresults.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages will become morereadily apparent from the following description, references being madeto the accompanying diagrammatical drawing, in which:

FIG. 1a is lateral view of the drilling tool according to the invention,respectively showing inner and outer inserts shown in solid lines;

FIG. 1b is a frontal view of the drilling tool according to FIG. 1a;

FIGS. 2a to 2e are respective lateral views of various indexableinserts, which can be inserted in the drilling tool;

FIGS. 3a-6a, are lateral views of the various embodiments of thedrilling tool according to FIG. 1; and

FIGS. 3b-6b are respective frontal views of the embodiments shown inFIGS. 3a-6a.

SPECIFIC DESCRIPTION

In FIGS. 1a to b a drilling tool according to the invention is shown towhose frontal shank side two indexable insert staggered by 180° aremounted. The hexagonal indexable inserts define together an apex angle γof 145°. Each of these indexable inserts has respectively cutting edges11, 12 and 21, 22, whereby however the cutting paths of the radiallyouter cutting edge 12 and the one of the radially inner cutting edge 21overlap. The radially outer indexable insert 20 determines the diameterof the bore, which is clearer defined by the bore wall 30. The radiallyinner indexable insert is arranged so that its radially inner cuttingedge 11 extends slightly beyond the drill axis, respectively thelongitudinal shank axis 40. In the embodiments of FIGS. 1a and 1b bothindexable inserts with a respective apex angle γ of 145° are inclinedwith respect to the longitudinal shank axis 40 by an angle α,respectively β of 20° respectively at 9°-10°. When the drilling tool isin use, at each cutting edge forces appear which can be taken upbasically vertically to the cutting edges. In the example of theindexable insert 10 these cutting forces are taken up as F₁ and F₂ andcan be decomposed into a vertical component parallel to the longitudinalshaft axis 40 and a radial component F_(r12) and F_(r11). Acorresponding situation results from a consideration with referencethereto of the radially outer indexable insert 20. In addition to theforces acting vertically with respect to the cutting edges, alsohorizontal cutting forces occur, which can be seen in Fig. 1b markedwith F₁₂, F₁₁, F₂₁ and F₂₂. While the horizontal cutting forces actingon the cutting edges 11 and 12 of the inner indexable insert areapproximately equal (FIG. 1b) and correspond approximately to thehorizontal cutting force F₂₂ acting on the cutting edge 22, thehorizontal cutting force F₂₁ is slightly smaller because of the clearlyshorter effective cutting edge 21. According to the concept of theinvention, the inclinations of the bisectors W1 and W2 of the twoindexable inserts 10 and 20 are selected so that the sum total of allradial forces F_(r11), F_(r12), F_(r21) and F_(r22) differs from zero.In this way, what remains with reference to all engaged cutting edges isa residual radial force component which together with all horizontalforce components forms a resulting total residual force F_(RES), whichdiffers from zero and which pushes the drill radially in the direction50 of the indexable insert 20 towards the outside, whereby a slightlylarger drilling diameter than the one corresponding to the radialdistance of the indexable insert 20 results. This insures that the drillcan not jam during its advance and also does not cause scoring whenwithdrawn. The angle δ formed by the total residual force F_(RES) withthe straight line 50 equals a maximum of ±45°.

Instead of the indexable inserts 10 and 20 with an apex angle γ of 145°,other indexable inserts can replace one or both indexable insertsaccording to FIG. 1. Such insertable indexable inserts are shown inFIGS. 2a to e with corresponding apex angles of 115°, 125°, 145°, 156°and 160° or a trochoidal cutting edge (FIG. 2e). The above listed apexangles correspond to a maximum bisector angle respectively equal to 8°,12°, 15°, 20° , or 25°.

An embodiment variant of the arrangement in FIG. 1 is illustrated inFIGS. 3a and b. As opposed to FIG. 1, the radially inner indexableinsert 10a is arranged so that their bisector W1 runs parallelly to thelongitudinal shank axis 40. Besides, the cutting edge 11 extends beyondthe drill middle by a far lager segment than the indexable insert 10 inFIG. 1a. The radially outer indexable insert, which has also an apexangle γ=145°, is inclined by approximately 3° with respect to thelongitudinal shank axis. The two cutting edges 21 of the radially outerindexable insert 20a as well as the outer cutting edge 12 of theradially inner indexable insert 10a overlap also clearly, so that inneither of the indexable inserts a radial balancing of the cuttingforces takes place. Besides it results here too that the sum total ofthe radial cutting forces differs from zero. The occurring horizontalforces F_(H1) and F_(H2) are shown as hatched areas, whereby a resultanthorizontal residual radial force component tending towards zero results,so that the residual radial force forms the total residual force FRESoriented in the direction of the indexable insert.

A corresponding force relationship is also achieved with the arrangementaccording to FIG. 4, wherein the radially inner indexable insert 10b isarranged so that its bisector W1 runs parallelly to the longitudinalshank axis 40. In opposition thereto, the radially outer indexableinsert 20b is inclined by an angle β of 5° with respect to thelongitudinal shank axis. The cutting path of the radially outer cuttingedge of the indexable insert 10b overlaps clearly with the cutting pathfollowed radially by the inner cutting edge of the indexable insert 20b.Thereby the effective cutting edge 21b is almost reduced to half.Correspondingly lower is also the corresponding horizontal cutting forcecomponent F_(21b). Thereby the horizontal components compensate eachother. Thereby from the radial force components of the indexable insert20b a common component is formed. An embodiment variant corresponding toFIG. 4 is shown in FIG. 5, whereby the apex angle of the used indexableinserts 10c and 20c each have 160°. The bisector W1 of the radiallyinner indexable insert 10c runs parallelly to the longitudinal shankaxis 40, while the bisector W2 of the radially outer indexable insert isinclined by an angle of 9° with respect to the longitudinal shank axis.

Generally it becomes clear from FIGS. 3, 4 and 5 that with an increasingapex angle of the indexable inserts and a constant angle α, theinclination angle β of the radially outer indexable insert has toincrease.

In all aforementioned embodiment examples the same indexable inserts(with identical apex angles) have been used. According to the embodimentshown in FIG. 6a, a corresponding effect can be achieved when as aradially outer indexable insert a trochoidal indexable insert 23 isused. The trochoidal indexable insert 23 is inclined so that its axis ofsymmetry W3 defines together with the longitudinal shank axis 40 aninclination angle β of 5°. Approximately the same inclination angle αforms the bisector W1 of the radially inner indexable insert 10d,whereby the cutting path of the trochoidal cutting edge 23 (see brokenline 23') clearly overlaps with the cutting path of the radially outercutting edge of the indexable insert 10d. Here too a residual radialforce component remains, which together with the horizontal residualforce component forms a general force acting on the cutting edge in thedirection of the radially outer indexable insert.

It is possible to use instead of indexable inserts of approximately thesame size with approximately equally long cutting edges, also indexableinserts of different size, whereby the relative load on the cuttingedges is lower, which has a wear-reducing effect on the cutting edges ofthe indexable inserts and also lowers the breaking risk of the indexableinserts. Here too the aim is to achieve an intended, strong overlappingof the cutting edge areas, which leads to the formation of residualradial force components, which according to the invention are equal butopposite to the horizontal residual radial cutting force components. Asfar as a remaining general resultant of radial forces and horizontalforces is accepted, this has in any case to be oriented in the directionof the radial line of an indexable insert.

As indicated in FIG. 5a, the radially inner indexable insert protrudesby a magnitude s, which does not surpass 0.15 mm.

What is claimed is:
 1. A drilling tool for drilling a bore in a solidmetal workpiece, said tool comprising:a shank having a central axis ofrotation and formed with a frontal end; a plurality of geometricallysimilar indexable inserts mounted on said frontal end and staggeredangularly equidistantly in a direction of a tool rotation from oneanother on said end, said inserts being spaced radially from saidcentral axis at different radial distances, so that at least one of saidplurality of inserts is an inner insert and another is an outer insert,each of said inner and outer inserts being formed with:a respective pairof contiguous cutting edges of a uniform length engaging a workpiece,defining thereby a respective working area therein which overlapsanother respective working areas defined by respective pairs of cuttingedges of other inserts, the cutting edges of the respective pairextending toward one another at an obtuse angle having a respective axisof symmetry, a respective engaged cutting edge of said pair of the innerinsert extending slightly beyond said central axis, at least one of theaxes of symmetry of the respective pair of cutting edges of said innerand outer inserts being inclined at a respective bisector angle notgreater than 25° with respect to said central axis and being inclined tothe axis of symmetry of the other of said inner and outer inserts at asector angle differing from 0°, all of the effectively engaged cuttingedges of said inner and outer inserts generating a resulting horizontalforce component which substantially equals zero upon rotation of thetool and a cumulative residual radial force component forming with saidresulting horizontal force component a residual force resultant directedradially toward the outer insert at a force angle differing therefrom bya maximum of 45°.
 2. The drilling tool defined in claim 1 wherein theobtuse angle is in the range of about 115° to 160° .
 3. The drillingtool defined in claim 2 wherein said obtuse angle is 160°, 156°, 145°,125° or 115° and corresponds to a maximum bisector angle respectivelyequal to 8°, 12°, 15°, 20°, or 25°.
 4. The drilling tool defined inclaim 1 wherein the bisector angle of the inner insert is smaller thanthe bisector angle of the outer insert.
 5. The drilling tool defined inclaim 1 wherein the bisector angle of the inner insert is zero.
 6. Thedrilling tool defined in claim 1 wherein a total length of the cuttingedges of the inner insert differs from a total length of the cuttingedges of the outer insert.
 7. The drilling tool defined in claim 1wherein the an effective cutting edge length of the outer insert islarger by 8% to 15% than an effective cutting edge length of the innerinsert.
 8. The drilling tool defined in claim 1 wherein the inner insertextends axially frontwardly beyond the rest of the inserts by a maximumof 0.15 mm.
 9. A drilling tool for drilling a bore in a solid metalworkpiece, said tool comprising:a shank having a central axis ofrotation and formed with a frontal end; a plurality of indexable insertsmounted on said frontal end and staggered angularly equidistantly in adirection of said rotation from one another on said end, said insertsbeing spaced radially from said central axis at different radialdistances, at least one of said plurality of inserts being an innerinsert formed with a respective pair of equally long contiguous cuttingedges extending toward one another at an obtuse angle and extendingslightly beyond said central axis with the respective engaged edge, sothat the cutting edges of the inner insert define a respective workingarea in the workpiece upon engaging the workpiece, another of saidplurality of inserts being an outer insert formed with a trochoidalcutting edge defining a respective working area overlapping the workingarea of the inner insert upon rotating the tool, said contiguous edgesand said trochoidal edge having respective axes of symmetry, at leastone of the axes of symmetry being inclined frontwardly toward saidcentral axis at a bisector angle not exceeding 25° and toward the otheraxis of symmetry at an angle differing from zero, all of the effectivelyengaged cutting edges of said inserts generating a resulting horizontalforce component which substantially equals zero upon rotation of thetool and a cumulative residual radial force component forming with saidresulting horizontal force component a residual force directed radiallytoward the outer insert at a force angle differing therefrom by amaximum of 45°.
 10. The drilling tool defined in claim 9 wherein theobtuse angle of the inner insert is in the range of about 115°-160°. 11.The drilling tool defined in claim 9 wherein the bisector angle of theouter trochoidal insert is maximum 20°.
 12. The drilling tool defined inclaim 9 wherein the bisector angle of the inner insert being smallerthan the one of the trochoidal outer insert.
 13. The drilling tooldefined in claim 9 wherein the bisector angle of the inner insert iszero.
 14. The drilling tool defined in claim 9 wherein the cutting edgesof the inner insert has a total effective cutting length differing froman effective cutting length of the trochoidal edge of the outer insert.15. The drilling tool defined in claim 14 wherein the effective cuttingedge length of the outer insert is larger by 8% to 15% than theeffective cutting edge length of the inner insert.
 16. The drilling tooldefined in claim 9 wherein the inner insert extends axially frontwardlybeyond the rest of the inserts by maximum 0.15 mm.